In current state-of-the-art microlithography objectives, particularly in immersion objectives with a numerical aperture value (NA) of more than 1.0, there is a growing need to use materials of a high refractive index. In this context, a refractive index is considered high if, at the given wavelength, it exceeds the value for the refractive index of quartz, which has a refractive index of about 1.56 at a wavelength of 193 nm. A number of materials are known whose refractive indices at DUV- and VUV wavelengths (<250 nm) are larger than 1.6, for example magnesium spinel with a refractive index of about 1.87 at a wavelength of 193 nm, or magnesium oxide which has a refractive index of about 2.0 at 193 nm.
When using these materials as lens elements, the problem presents itself that due to their cubic crystallographic structure, they exhibit a degree of intrinsic birefringence that increases as the wavelength becomes shorter. For example, measurements of the retardation due to intrinsic birefringence in magnesium spinel at a wavelength of 193 nm produced a value of 52 nm/cm, while the retardation due to intrinsic birefringence in magnesium oxide at a wavelength of 193 nm was estimated to be about 72 nm/cm. Depending on the design-related conditions in the image field, a retardation of this magnitude can lead to lateral ray deflections that are three to five times as large as today's critical structure widths of about 80-100 nm.
As a means to reduce the negative effect on the optical image caused by intrinsic birefringence in fluoride crystal lenses, it is known for example from US 2004/0105170 A1 and WO 02/093209 A2 to arrange fluoride crystal lenses of the same crystallographic cut in rotated orientations relative to each other (a concept known as “clocking”) and, in addition, to combine several groups of such arrangements with different crystallographic cuts (for example groups of 100-lenses and groups of 111-lenses) with each other.
Although the negative effect of the intrinsic birefringence can be compensated by this method to a certain extent even in the aforementioned highly refractive cubic materials, a further problem presents itself in that the compensation achieved with the aforementioned “clocking” is incomplete in the case where the respective “compensation paths” are different (i.e., the respective path lengths which the rays that enter into interference with each other traverse in the mutually rotated parts of the same crystallographic cut). This is the case in particular in a projection objective that produces an off-axis image field. Off-axis fields of this kind are present in particular in catadioptric projection objectives with geometric beam-splitting of the type disclosed, e.g., in WO 2004/019128.
The aforementioned problem with different compensation path lengths in different materials used for the compensation of birefringence can also occur in materials with natural birefringence, for example if materials with opposite (positive/negative) signs in their birefringence are combined with each other for compensation, as described in WO 2005/059645, or with the “clocking” of materials with natural birefringence.